0 votes 0 votes Show that $Z^{+} \times Z^{+}$ is countable by showing that the polynomial function $f : Z^{+} \times Z^{+}\rightarrow Z^{+}$ with $f(m, n) = \dfrac{(m + n − 2)(m + n − 1)}{2} + m$ is one-to one and onto. Set Theory & Algebra kenneth-rosen discrete-mathematics set-theory&algebra descriptive + – admin asked Apr 21, 2020 • edited Apr 21, 2020 by Lakshman Bhaiya admin 223 views answer comment Share Follow See all 0 reply Please log in or register to add a comment.